1. Field of the Invention
The invention pertains to a flux composition and to a corresponding method for soldering, for example, a semiconductor chip or a chip carrier module to a printed circuit board.
2. Description of the Related Art
Fluxes play an important role in the procedures used to mount electronic components onto printed circuit cards and printed circuit boards (both of which are hereinafter generically referred to as printed circuit boards or PCBs). For example, one method for directly mounting a semiconductor integrated circuit device (hereinafter denominated a semiconductor chip or just a chip) onto a PCB is, for example, to form regions of solder, e.g., solder balls, on contact pads on the circuit-bearing surface of the chip. Such solder regions may also be formed on corresponding contact pads on the PCB. Then, a flux is applied to the solder regions on the chip and/or to the corresponding contact pads and/or corresponding solder regions on the PCB in order to remove oxide layers which may have formed on these solder regions or contact pads and to achieve increased wetting of the contact pads by the solder regions. Thereafter, with the circuit-bearing surface of the chip facing the PCB, the solder regions on the chip are brought into contact with the corresponding contact pads or solder regions on the PCB, and the resulting assembly is heated in order to melt, and thereby reflow, the solder regions on the chip and/or on the PCB. Upon cooling and re-solidification, the resulting solder connections between the chip and the PCB are typically encapsulated in an encapsulant, e.g., an epoxy resin encapsulant, to relieve any strains which may be engendered by a difference between the coefficient of thermal expansion (CTE) of the PCB and the CTE of the chip.
In a manner similar to that described above, one method for mounting a module, e.g., an organic module or a ceramic module, bearing semiconductor chips (hereinafter denominated a chip carrier module or just module) onto a PCB, involves forming, e.g., screening, regions of solder onto contact pads on the non-chip-bearing surface of the module. Such solder regions may also be formed on corresponding contact pads on the PCB. A flux is then applied to the solder regions on the module and/or to the corresponding contact pads and/or corresponding solder regions on the PCB. Thereafter, with the non-chip-bearing surface of the module facing the PCB, the solder regions on the module are brought into contact with the corresponding contact pads or solder regions on the PCB and the resulting assembly is heated in order to melt, and thereby reflow, the solder regions on the chip and/or on the PCB. In general, the magnitude of the difference between the CTE of the module and the CTE of the PCB is relatively small, and therefore the resulting solder connections between the module and the PCB need not be encapsulated in an encapsulant.
If the module of interest has electrically conductive pins extending from the non-chip-bearing surface of the module, then the module is mounted onto a PCB by, for example, initially positioning the module over the top (i.e., the circuit-bearing) surface of the printed circuit board and inserting the electrically conductive pins of the module into corresponding, copper plated through holes (PTHs) extending through the thickness of the PCB. Then, the PCB and the module are placed on a conveyor, which passes the PCB and module over a fluxing wave or flux sprayer, which serves to impinge liquid flux onto the bottom surface of the PCB and into the PTHs. This flux is wicked up into the PTHs, and thus flux is applied to both the walls of the PTHs and to the pins extending into the PTHs. Thereafter, the conveyor passes the PCB and module over a solder wave, which serves to impinge liquid solder onto the bottom surface of the of the PCB and into the PTHs. This liquid solder is also wicked up into the PTHs, filling the PTHs and, upon cooling and solidification, serving to encapsulate the pins within the PTHs.
One of the most important aspects of the above-described chip-mounting and module-mounting procedures is the choice of flux. That is, as noted above, the flux serves to remove any oxide layers which may have formed on the solder regions, contact pads, pins or PTHs and to increase the wetting of, for example, contact pads by solder regions. In most instances, at the completion of the soldering process, use of the commonly available fluxes results in ionic residues remaining on the solder regions, contact pads, pins or PTHs. Such ionic residues are undesirable because they lead to corrosion of circuitry and to short circuits. Consequently, if formed, such ionic residues must be removed, e.g., cleaned with water, after the completion of the soldering process.
The solder connections formed between a chip and a PCB or between a pinless module and a PCB, as described above, have relatively small heights, e.g., 4 mils, and therefore the spacing between a chip or pinless module and its PCB is correspondingly small. This is significant because it implies that it would be very difficult, if not impossible, to clean away any ionic residues remaining on the solder regions and/or contact pads after the completion of the soldering process. In addition, in the case of a pinned module, while corresponding ionic residues are readily cleaned with water, one must then deal with the environmental hazards posed by the resulting contaminated water.
Significantly, those engaged in the development of fluxes and soldering processes for mounting chips and modules onto PCBs have sought, thus far with little success, fluxes which leave essentially no ionic residues on solder regions, contact pads, pins or PTHs at the completion of the corresponding soldering processes.